The majority of commercial photovoltaic modules based on silicon solar cells utilise solar cells fabricated using high quality silicon wafer. Despite the constant drop of silicon prices over the last few years, the cost of silicon still represents a large portion of the final price of these photovoltaic modules.
Substantial investments have been made in the last 10 years to develop photovoltaic devices which use inexpensive materials, possibly in very small quantities. These photovoltaic devices are often referred to as thin film solar cells. Cadmium telluride cells are an example of a thin film solar cell technology which had a major commercial success and competes on the marked with conventional wafer-based silicon cells.
Some thin film solar cell technologies are based on organic photoactive layers (OPVs). Generally these layers are realised using solution-processable materials and can be fabricated over large areas on rigid or flexible substrates. Potentially OPVs may become as inexpensive as paint in terms of processing and fabrication cost.
Critical challenges faced by OPV technology relate to the low power conversion efficiency (PCE) of devices. The PCE of current single cell OPV device is around 8%. This low value of efficiency is due to the inability of OPVs to convert a large part of the solar spectrum, given the narrow absorption range, and the low values of carrier mobility, which forces device to have a thin absorbing layer.
A possible approach to increase the PCE of OPVs is to utilise an OPV cell in synergy with another photovoltaic device in a stack configuration. This technique has been adopted, for example, with some silicon based thin film solar cells. Using two or more cells in a stacked configuration allows exploiting the solar spectrum and improving the overall performance of the device.
However, OPVs have peculiar chemical and structural properties and the challenge has been to develop performing photovoltaic materials that can be used together with OPVs reliably.